The present invention relates to an apparatus and method for optical multimode receiver power stabilization and protection particularly in free space optical systems.
Free Space Optical (FSO) propagation results in large fluctuations of optical receive power. The fluctuations can result in bit errors in optical receivers where it is desirable to stabilize the optical power and establish the best signal to noise ratio prior to detection.
In single mode systems optical power stabilization can be accomplished with active low noise optical amplification with an optical gain that can be rapidly adjusted to negate the optical power fluctuations prior to detection using a PIN photodiode. In multimode systems, however, low noise, optical amplification is very difficult to achieve requiring selection of the most sensitive optical receiving device available to try to obtain the best signal to nose ratios during signal fades.
The readily available optical receiving device is the Avalanche-based Photo Diode (APD) receiver. These devices are operated at high voltages, typically in the 60V range, near the point of avalanche break down. However, a risk in using these devices with systems that have rapid fluctuations in received optical power is that cases of excess optical power in combination with the high electrical bias to the APD can very easily create the avalanche current condition and destroy the detector or follow on electrical amplifiers.
A system level design is required to ensure protection of these sensitive receivers under various conditions. A variable optical attenuator (VOA) in front of the APD can be used to protect it from high optical power situations during normal operation. However, even with a VOA responding to power fluctuations, there are several conditions that must be protected against.
In a startup condition, when the DC power is turned on, the system initial conditions tend to start from minimal attenuation, as the typical control loop will be starting from a zero condition. If optical power is present at startup, this can result in excess power being applied to the photoreceiver.
Similarly, when the optical link goes down, the receive system will drive the attenuators to a minimal attenuation state due to lack of power. If the link rapidly comes back online, a power spike can easily make it through the VOA, as the control system has a finite response time.
Therefore, at both DC power-up as well as the link outage stages of system operation, attention must be paid to provide opportunities to protect the sensitive photoreceiver from a potential over power condition. This over power condition must be implemented by the system controller when exposed to a link outage or initial power up condition to protect the receiver.
Also, vital to system operation is the correct sensing and control of optical power. Multimode device performance can vary dependent upon the excited modes contained in the fiber. The performance of multimode devices to different modes is usually not specified; often the only specification that is provided is to the fundamental mode or the case when all modes are excited.
In FSO applications, all modes are often not excited which results in unexpected or unknown behavior from multimode devices. For example, this could result in varying coupling ratios in optical couplers and varying attenuation in optical attenuators dependent upon the modes present. Specific designs that are non-modal dependent are found to be a critical choice in a Free Space Optical receiver system design for proper performance.
In order to stabilize optical power, the system must have an accurate measure of optical power, and have a precise ability to control the power when setting the attenuator. Various technologies for implementing optical couplers exist, but not all can consistently maintain their specified coupling ratio for various mode loading conditions. One type of couplers that have been proven to work optimally for multimode applications are thin-film couplers.
Similarly the attenuator design must also provide consistent attenuation with varying mode structure. For example, in MEMS based VOAs, the beam inside the attenuator is either partially blocked or mis-pointed to decrease the power level through the device. Depending on which modes are excited, the attenuation levels vary leading to unexpected system performance.
In multimode systems, amplification is not yet a readily available option, so variable optical attenuation must be used for power stabilization. Multimode optical attenuators can have highly temperature dependent non-monotonic nonlinear attenuation responses versus drive signal. Since a control system prefers a linear function, such devices create excess demands on the signal processor. What is needed then are an apparatus and method to handle these optical devices in an efficient manner to allow a standard embedded processor to control the devices with an efficient computation so control loop times can be maintained to provide an acceptable system response time.